The invention relates to an alternating current ignition system with at least one ignition output stage, comprising an ignition coil with a primary and a secondary winding, a semiconductor switch connected in series to the primary winding, a resonant-circuit capacitor that provides a resonant circuit for generating a bipolar alternating current with the primary coil, and an energy recovery diode arranged in parallel to the semiconductor switch. An alternating current ignition system of this kind is known from DE-OS 39 28 726 and, compared with conventional ignition systems such as, for instance, the so-called transistor ignition systems with inactive high-voltage distribution, has the advantage that small and consequently low-cost ignition coils can be used. As a result, the time of ignition is reached quickly, within a matter of microseconds. Furthermore, according to the above-mentioned publication, optimum ignition is ensured by it remaining in the switched-on state for the entire period of combustion irrespective of the engine speed and during which it generates a bipolar sparking current.
An alternating current ignition system of the kind known from the above-mentioned publication is shown in FIG. 1. In this Figure, reference character Z designates an ignition output stage that has an ignition coil Tr with a primary and a secondary coil, a semiconductor switch T connected in series to the primary coil, as well as a resonant-circuit capacitor C and an energy-recovery diode D that are also arranged in series to the primary winding. Also in series with the semiconductor switch T, there is a current measuring resistor R1 for detecting the actual value of the primary coil current. A control circuit 1 controls the semiconductor switch T through its control electrode for which purpose the voltage drop across the resistor R1 and the voltage U.sub.T across the semiconductor switch T is supplied via the circuit junction point A. A control signal containing the ignition signal is supplied to the control circuit 1 via its connection U.sub.st. A switched-mode power supply not shown in FIG. 1 generates an operating voltage U.sub.B of 180 V that is applied to the primary coil of the ignition coil Tr. The switched-mode power supply is in turn supplied from an on-vehicle battery.
The ignition output stage Z is operated in Current Mode, i.e. the semiconductor switch T is switched on until the current flowing through the primary coil reaches a specific value and then the semiconductor switch T switches off so that the energy stored in the primary coil can charge the capacitor C. This leads to an approximately sinusoidal variation of the voltage applied at the semiconductor switch T and at the same time the negative half-wave of the oscillation is limited by diode D to small voltage amplitudes. During this phase of current flow through diode D, the semiconductor switch T should again be switched on. At this moment, the switch-on losses are also very low because the voltage applied to the semiconductor switch T has a value that is very nearly zero.
The actual value of the current flowing through the primary winding is normally measured through the voltage drop across the resistor R1. When the current has reached its command value, the semiconductor switch T is switched off and consequently the voltage across the resistor R1 decays very rapidly. In order to prevent the semiconductor switch T from switching on again immediately, various measures are known.
One of the known measures involves evaluating the voltage U.sub.T on the semiconductor switch T. In accordance with FIG. 1, this is accomplished by the junction point A of the semiconductor switch T together with the winding of the ignition coil Tr being connected to the control circuit 1 where it is evaluated. This solution has the disadvantage, however, that the next switching-on operation can be prevented only when the voltage U.sub.T has reached a value that is greater than the supply voltage U.sub.B. Therefore, in order to prevent oscillations from occurring during the time until the voltage U.sub.T has reached the value of the supply voltage U.sub.B, an additional disabling means, such as a timing element, must be used. Another disabling device of this kind must also be used if the voltage U.sub.T at the semiconductor switch T again drops below the value of the supply voltage U.sub.B in order to obtain the above-mentioned advantage of switching at a voltage level of almost zero. The disadvantage of such a simple type of timing element, however, is that the switch-off threshold of the primary current is affected. Where there are several primary circuits, a further disadvantage is that the voltages U.sub.T generated at the semiconductor switches T must be measured at least once per primary circuit, even if the evaluation of the primary currents takes place only once for the entire ignition system.
In another known solution, a monostable flip-flop (mono-flop) is used in order to prevent the semiconductor switch T from switching on again for a defined period of time. This solution with a defined time delay has the disadvantage that the time delay to be selected is firstly a function of the selected primary current and secondly it also depends on whether the breakdown of the spark gap on the secondary side of the ignition coil has already taken place or not. Finally, the tolerances of all time-determining components are included in the time delay to be selected. Consequently, this solution cannot in all cases guarantee reliable operation of the output stage.